Recently, demand for liquefied natural gas (LNG) has markedly increased, and thus, there is increasing demand for transportation equipment and storage tanks for transporting and storing extremely low temperature LNG. Tanks for transporting and storing LNG should have sufficient impact resistance at LNG temperatures equal to or lower than −162° C. Typical materials having a high degree of impact resistance at extremely low temperatures are aluminum (Al), 9% nickel (Ni) steel, and stainless steel (hereinafter referred to as “STS”).
However, aluminum (Al) has a low degree of tensile strength, and thus relatively thick aluminum plates have to be used. In addition, aluminum (Al) has poor weldability.
9% Ni steel requires relatively expensive welding materials such as Inconel 625 (nickel: 50 wt % or more, Cr: 20 wt % or more), and the yield strength of welded joint of 9% Ni steel is low. STS is relatively expensive and has a low degree of thermal strain, and the quality of STS is not guaranteed at extremely low temperatures.
Therefore, it is necessary to develop a technique for forming high manganese (Mn) welded joints for use in extremely low temperature environments by using a material guaranteeing weldability and which is relatively inexpensive, as compared to nickel (Ni) used as an austenite stabilizing element.
Particularly, a welded joint having a degree of impact toughness of 27 J or greater is necessary for maintaining the stability of a structure at an extremely low temperature of −196° C. or lower, and a welded joint having a room temperature yield strength of 360 MPa or greater is necessary for manufacturing a structure at room temperature. To this end, in the related art, materials containing large amounts of relatively expensive nickel (Ni) and chromium (Cr) (Ni: 50 wt % or more, and Cr: 20 wt % or more) are used to form welded joints. Therefore, there is a need for a welding material containing low amounts of relatively expensive elements and a welded joint formed of the welding material.